Stationary and transport applications of air conditioning and refrigeration systems, with transport applications including those systems used with straight trucks, tractor-trailer combinations, refrigerated containers, and the like, conventionally utilize a chlorofluorocarbon (CFC) refrigerant in a mechanical refrigeration cycle. The mechanical refrigeration cycle requires a refrigerant compressor driven by a prime mover, which often includes a dedicated internal combustion engine, such as a diesel engine. Because of the suspected depleting effect of CFC's on stratospheric ozone (O.sub.3), practical alternatives to the use of CFC's are being sought.
The use of a cryogen, i.e., a gas which has been compressed to a very cold liquid state, such as carbon dioxide (CO.sub.2) and nitrogen (N.sub.2), in a transport refrigeration system is particularly attractive because, in addition to eliminating the need for a CFC, it also eliminates the need for a compressor and associated prime mover. Refrigeration systems of which we are aware which utilize a cryogen, implement a cooling cycle by circulating the cryogen through a fluid path which includes a heat exchanger disposed in heat exchange relation with air from a conditioned space. When a heating cycle is required to hold the temperature of the conditioned space within a temperature range close to a selected set point temperature, or a defrost cycle is required, the cryogen is heated by a suitable burner and combustible fuel, and the heated cryogen is circulated through the fluid path. Thus, cryogen is expended to the atmosphere during a cooling cycle, and cryogen plus a fuel associated with the heat source, such as propane, liquid natural gas, diesel fuel, and the like, are expended to the atmosphere to implement heating and defrost cycles.
It would be desirable, and it is an object of the present invention, to provide a new and improved cryogenic based refrigeration system, which more effectively and efficiently utilizes a cryogen, for lower cost operation, as well as for an extended operating time for a given vessel of cryogen.